The World Radio Conference in 1997 (WRC-97) allocated NGSO satellite systems co-primary status with GSO satellite systems in certain frequency bands based on provisional limits on the Effective Power Flux Density (EPFD) that the NGSO systems produce. Studies continue to refine these EPFD limits to allow the GSO and NGSO systems to operate simultaneously sharing the Ku-band of the frequency spectrum. The interference mitigation technique of the present invention reduces the peak interference levels for the GSO ground station antenna and allows the NGSO system to meet the proposed limits more easily. The technique of the present invention produces interference that is more tolerable to GSO systems than interference produced by the techniques currently planned for use by LEO NGSO satellite systems. Interference that does occur with the technique of the present invention is primarily (if not wholly) outside the “short term interference” band of the EPFD distribution curve. In this “short term interference” band, for example, the interference may be −40 dB below the limit.
A satellite in a geostationary or geosynchronous orbit (GSO) is positioned above the equator at an altitude of about 35,800 km and at an inclination of about 0 degrees. A GSO satellite orbits the earth once per day in synchronous motion with the revolution or rotation of the Earth. The satellite appears fixed in the sky to an observer on the surface of the earth. Communicating with a GSO satellite has some obvious advantages in that an earth station antenna can remain pointed in one stationary and fixed direction without the need for active control to maintain pointing at the GSO satellite. A GSO satellite provides coverage of only a portion of the earth and cannot cover the Polar regions, which are 90° in arc away from the plane of the satellite. Additionally, the round-trip time delay between an earth station and the GSO satellite is relatively large, which can have undesirable effects on communications.
NGSO satellite constellations have been proposed to overcome some of these problems. A constellation of NGSO satellites can provide complete global coverage including the Polar regions, because such NGSO constellations can include satellites at inclination angles other than 0°. Since NGSO satellites are nominally at a lower orbit altitude, the round-trip time delay will be lower.
One of the early NGSO orbit constellation proposed for spectrum sharing with GSO satellite systems was a Low Earth Orbit (LEO) satellite system of up to 80 satellites. This constellation was called (SkyBridge—USAKUL1). By its nature, a LEO NGSO satellite will spend a significant amount of its orbit period in-line between a GSO satellite and a point on the earth where there might be a GSO earth station. (FIG. 1) The result is that some interference mitigation approach or avoidance technique is required to avoid interference with operation of the existing GSO satellite network. In the approach adopted by SkyBridge, the NGSO satellite turns off only the antenna spot beams that service an area that would be in an alignment condition between a GSO satellite and a GSO earth station. This SkyBridge technique avoids main-beam to main-beam interference and limits the interference to the GSO earth station receiver. A disadvantage of the SkyBridge technique is that satellite antenna spot-beams that are not serving the affected area would still be operating, and sidelobes from these spot-beams could cause significant interference to the GSO earth stations, particularly for earth stations with large antenna apertures. Such interference would be relatively significant, especially in the “short term interference” band, and would be disruptive to the GSO communication system.
The Ellipso system proposed a Highly Eccentric Orbit (HEO). While the orbits proposed for SkyBridge is circular, and the satellite is always at the same altitude above the earth, a HEO orbit would use an eccentric ellipse having the center of the Earth as one foci of the ellipse. As a result the satellite altitude varies significantly over its orbit. For orbit stability, a HEO satellite operates at a high inclination, and the satellite only communicates during the high altitude portion of the orbit. As a result of the typical operating characteristics of the HEO orbit, there is a large discrimination angle between the HEO satellite and a GSO satellite. No in-line condition occurs (NGSO satellite in-line with a GSO satellite and a GSO earth station), which significantly reduces the peak interference into any of the GSO earth stations. The Ellipso system is more completely described in U.S. Pat. Nos. 5,669,585; 5,788,187; 5,845,206; and 5,979,832, which I incorporate by reference.
Individual satellites of a HEO satellite constellation are only operating during a portion of their orbit, increasing the number of satellites required to provide continuous communication coverage. The additional satellites impose significantly higher start-up and capital costs for the system. A Molniya HEO orbit, for example, requires three satellites. Each operates for only 8-hours of the orbit to provide 24-hour coverage. HEO satellites also transit the Van Allen radiation belts continuously. The satellite therefore has to be significantly radiation hardened, making it more costly. The transit through this intense radiation lowers the expected lifetime for each satellite. Because the satellite altitude is constantly changing during its operating period, the service area covered by a typical satellite reflector antenna will also constantly change unless active beam pointing and beamwidth control is used. Adding beam pointing and beamwidth control systems to each satellite requires a more complex and costly antenna control system.
The system Ellipso described in U.S. Pat. No. 5,979,832 involves an array of satellites that looks like a planetary gear system. The satellites are in low to medium earth orbit (LEO to MEO) in two interactive orbital rings. An outer ring contains circular orbit satellites. An inner ring contains elliptical orbit satellites. The apogees of the elliptical orbits are approximately tangential to the diameter of the circular earth orbits. The periods of the two rings are adjusted to be proportional to the numerical ratio of the number of satellites in one ring with that of the other. The adjustment allows the elliptical inner ring of satellites to be spaced always midway between the satellites (or “teeth”) of the outer ring for a specified parameter. This spacing can be tailored to a specific point on the earth or to a given time of day. The spacing between satellites in the “planet gear” constellation will be approximately equal anywhere in the populated world during daytime hours. Nighttime coverage is likely less critical since fewer people will be using resources at night—more people are sleeping. Hence, the circular satellites are presumed to be capable of handling the nighttime traffic alone, without involving the elliptical satellites. The fact that the inner elliptic ring of satellites overtake and pass the outer circular ring of satellites on the nighttime side of the earth is thus not a cause for serious concern, provided that the assumption of a decline in load actually occurs. If messages are stored and transmitted prefecentially at “off-peak” hours, the relative amplitudes of the peaks and troughs in usage, however, would be reduced and load might approach a steady state. This HEO system might encounter problems with a steady state pattern of use.
“Apogee pointing toward the sun” (APTS) satellites useful in the Ellipso elliptical orbits are described in U.S. Pat. No. 5,582,367, which I also incorporate by reference.
A Medium Earth Orbit (MEO) constellation of the present invention can provide the advantages of the HEO system (large discrimination angles) in reducing the overall interference to the GSO satellite networks without the disadvantages of the HEO system. The interference to the GSO satellite networks can be significantly reduced over that produced by the LEO systems without the inefficiencies (i.e., need for a larger number of satellites), complexity, or cost of the HEO systems.
A MEO satellite system is described in U.S. Pat. Nos. 5,433,726; 5,439,190; 5,551,624; and 5,867,783, which I incorporate by reference. Such a system would be able to provide complete global coverage, including the Polar regions, which the GSO satellite networks cannot do.
A MEO/GSO satellite system is described in U.S. Pat. No. 5,971,324, which I also incorporate by reference.
U.S. Pat. No. 6,011,951 describes an interference avoidance technique for two Teledesic LEO constellations sharing a common radio frequency band. A first and a second satellite communication system each contain a plurality of satellites in a plurality of non-geostationary (NGSO) Earth orbits. Each of the plurality of NGSO satellites has a predefined orbital plane. Within each orbital plane, satellites of the first and second satellite communication systems are alternating, such that each orbital plane contains one or more satellites from both of the satellite systems. In this manner, it is possible to achieve satisfactory discrimination between satellites and Earth-based stations. The Earth-based station of each communication system will communicate with the closest satellite of its respective communication system. In an alternative technique that is particularly useful when an Earth-based station in the first communication system is able to communicate with more than one satellite, a satellite is selected based on the topocentric separation of the satellite from satellites in the second system. The system can also combine alternating satellites within an orbital plane with alternating orbital planes with satellites of each respective communication system.
A need remains for a technique to limit interference between a MEO constellation and a GSO system. The present invention provides such a technique.
A need also remains for a low-cost, reliable satellite architecture suitable for use in a MEO system to provide global communication coverage to and from mobile platforms, such as airplanes, trains, ships, or automobiles. Such a satellite constellation would allow access to the Internet, for example, to a global population with increasing frequency to be “on the move.”
These and other features of the present invention will now be described in the Summary and Detailed Description.